Throughout history, many nations have tried to destroy the Jewish people. However, we are commanded to permanently remember the attack of only one of these nations; that of Amalek, when they attacked the Jewish people shortly after the splitting of the Sea.
In order to gain a deeper understanding of the danger that Amalek poses to the Jewish nation, it is instructive to closely analyze the commandment to remember Amalek's heinous deed: "Remember what Amalek did to you, on the way when you were leaving Egypt; that he happened upon you on the way. and he struck those of you who were hindmost, all the weaklings at your rear, when you were faint and exhausted, and he did not fear G-d."
The majority of the verse focus on Amalek's despicable actions, such as how they attacked us when we were weak and tired. However, the end of the verse points out the main negative characteristic that Amalek displayed - that they did not fear G-d. Rav Moshe Sternbuch shlita notes that Amalek are known to be the epitome of evil, and surely possess numerous terrible traits and beliefs. Accordingly, he asks that it is difficult to understand why the Torah focused in particular on the seemingly mild flaw of lacking in ‘yiras Shamayim’?
He explains that the Torah is teaching us that the root cause of Amalek’s evil character was his lack of yiras Shamayim. Why is this the case? One significant aspect of Yiras Shamayim is that one who fears G-d is aware of Hashem’s involvement in the world. He sees Divine Providence in everything that takes place. But a mere awareness does not suffice; he takes this awareness and uses it to understand how HaShem is communicating to him. This greater recognition brings one who fears G-d closer to fulfilling His will.
In contrast, one who lacks yiras Shamayim is blind to the events around him. He does not see G-d’s hand in the most miraculous events, rather he irrationally ascribes it to the random laws of nature. Thus, he is not moved by anything, no matter how remarkable. Such a person will never come closer to the truth because nothing effects him. Amalek epitomized this trait. They were aware of the remarkable miracles of the Ten Plagues and the splitting of the Sea, yet they paid no attention to the logical consequences of these events - that there is an All-Powerful Being who was guiding the Jewish people. They refused to recognize any sense of uniqueness about the Jewish people and flagrantly attacked them. In this way, their lack of ‘yiras Shamayim’ was the source of their evil actions.
This idea is further described by the Torah: It explains how Amalek “happened upon” the Jewish people. The hebrew word used here is ‘korcha’. Chazal teach us that the root of korcha is similar to the word for ’cold’ - ‘kor’ - Amalek cooled down the world’s fear of the Jewish people that they felt after the miracles of yetsias Mitzrayim. They bring an analogy of a boiling hot bath, that is so hot that no person can go inside. Then, one person jumps inside it. He burns himself but he cools it down for the other people to be able to go in it. Similarly, the non-Jewish nations were attack to fight the Jewish people after all the miracles that they had experienced. Amalek paid no heed to these miracles and attacked. Even though they greatly damaged themselves, they also reduced the fear of the other nations towards the Jews.
Why did Amalek respond differently from the other nations, to the miracles of yetsas Mitzrayim. The non-Jews worshipped false G-ds but they believed in the idea of a power guiding a nation. Accordingly, they believed in the 'G-d of the Jews' and paid heed to His protection of the Jewish people. Amalek, in contrast, seem to have been atheists. They believed in no force, therefore they attributed all of the wondrous events of yetsias Mitzrayim to chance. Accordingly, they could ignore all the signs and jump into the boiling bathtub.
We have seen that the root of Amalek's evil was their belief in the randomness of events and the accompanying total rejection of a Higher Being. This caused them to react 'coldly' to everything that they witnessed, and even to cause other nations to 'cool down' their fear of the Jewish people. This attitude is something that is unique to Amalek amongst all the nations, and in a certain sense, poses more of a danger to Torah observance than the idolatrous beliefs of the other nations. It causes 'believing' Jews to lose their sense of wonder about the miracles that surround them, and to even subconsciously attribute them to chance. Moreover, it prevents a person from learning from events around him, making him immune to the lessons that Hashem sends him. in this vein, Rav Sternbuch writes that a person who merits to see the salvations of HaShem and His wonders, yet remains are blind to what goes on around him, and is not aroused to fear Hashem; he should know that he is surrounded by impurity and is under the influence of Amalek.
When we read Parshas Zachor we should focus on the lack of yiras Hashem that characterized Amalek. Through this contemplation may we merit to remove the power of Amalek from the world.
 Ki Seitsei, 25:17-18.
 Taam v’Daas, Devarim, Ki Seitzei,25:18, p.163. Also see Ayeles Hashachar of Rav Aryeh Yehuda Leib Shteinman shlita, Ki Seitzei, 25:18, for a similar approach.